Prochain séminaire

Résumé: Acousto-optic imaging coupled with ultrasound modality would be able to discriminate between healthy or diseased biological tissues thanks to the additional optical contrast. Acousto-optic imaging is a multi-wave technique which localizes light in very scattering media with acoustic waves. The main issue is the detection of the localized light. In this presentation the investigated solution is the use of a spectral filter based on spectral hole burning. The goal is to use a YAG crystal doped with thulium ions under a magnetic field, which increases the lifetime of the spectral hole from 10ms to longer than a minute.
In this presentation I will first give an overview of the context of my work. Then I will present the acousto-optic images achieved with a long-lived spectral filter in Tm:YAG, in a scattering medium. Lastly, I will also raise the subject of the challenges specific to the addition of magnetic field to my setup.

Résumé: Spatial Modulation Multiple Input Multiple Output (SM-MIMO) appeared to address both the needs of growing data rate and energy-efficient of smart devices for Internet-of-Things (IoT) and wireless networks (5G, Wi-Fi, etc.…) applications. In the first part of the presentation, we propose to take benefit of coupling effects between resonators to generate different radiation patterns that stand for SM-MIMO symbols. The first reconfigurable antenna results from the coupling of meander monopoles and L-shaped resonators are discussed. The experimental prototype generates efficiently between 4 and 8 patterns. The concept of spatial diversity that is a key feature in SM-MIMO is discussed. The second reconfigurable antenna is a metamaterial-like structure based on split ring resonators (SRR). A semi-analytic model that describes the magneto-electric coupling between SRR is developed in agreement with the numerical and experimental results proposed model. In a close collaboration with IETR, these antennas have been successfully implemented in a SM-MIMO device to transmit information.
Most of the wireless communication devices used an active antenna to transmit information. However, researchers have been investigating the potential of backscatter communications to take the benefit of the ambient electromagnetic waves by modulating them with a backscatterer to a reader. This technique allows to lower down the power consumption just by recycling the ambient electric fields (DVB-T, 4G, etc…). In the second part of the presentation, we show how the modulated field is strongly sensitive of interference effects between the direct path and the scattered one. Within a collaboration with Orange Labs, we propose a systematic survey of this effect of interferences. Fading patterns are observed even in a Line of Sight (LOS) configuration. Experimental results are validated by an analytical model. Finally, it is demonstrated that the Binary Error Rate (BER) is directly driven by these interferences.

Résumé: A wireless contact lens, incorporating various functions such as data transfer, energy collection, eye movement extraction, is presented. It measures the direction of gaze, vergence, blinks and jerks, combining optical and RF technologies. Their implementation is discussed from flexible substrates, encapsulable in scleral lenses, such as micro-batteries and graphene-based biofuel cells. Preliminary results will be presented.

Résumé: We utilize a generalization of the Wigner-Smith time-delay operator to manipulate a target embedded inside a disordered structure by shaping the incident wavefront. Such a manipulation involves, e.g., applying a well-defined torque onto the target or strongly focusing onto it. Our technique relies on the generalization of the Wigner-Smith time-delay operator and was successfully tested in a microwave setup featuring a waveguide containing a disordered medium.

Résumé: Since 2012, the field of deep learning has seen numerous advances in various domains
(computer vision, speech recognition, natural language processing, etc.), often at a fast pace.
These evolutions often originate from paradigmatic developments in new neural network
architectures and layers. The purpose of this presentation is to give an overview of the
recent developments in deep learning by focusing on their constitutive bricks,
i.e. the layers, and the way they are assembled in various models.
As a concrete example of deep learning model building, we will study the recent participation of
Philips Research France (Medisys Lab) to the JFR (Journées Francophones de Radiologie) 2018 challenge on knee meniscus.
The goal of this challenge was to detect and classify tears in knee meniscii on MRI images.
The retained solution, which brought the first place to the Philips/Hospices Civils de Lyon team,
is a combination of image segmentation and classification models, with the addition of special image pre-processing.

Résumé: Nanophotonic structures are capable of generating field hotspots, which can enhance quantum light-matter interactions by many orders of magnitude. However, numerical simulations for applications such as radiative heat transfer, electron energy loss spectroscopy, van der Waals forces, Purcell factor throughout a volume, and many others are challenging and often computationally prohibitive. Common to these simulations is that the Green’s function or local photonic density of states must be known at each point across a volume of space, necessitating the solution of Maxwell’s equations perhaps many thousands of times.
We propose a modal solution, which requires just a single simulation to find the modes of the nanophotonic system, from which we immediately obtain the Green’s function everywhere in space. This not only reduces simulation time by approximately 2 orders of magnitude, but also offers ready physical insight into the spatial variation of Green’s function. Modal methods have long been used for closed systems, where the formulation is exceedingly simple. We have generalized modal methods to open systems while maintaining this simplicity, catering to the explosion of research interest in nanophotonics. We furthermore present a highly-efficient exponentially-convergent method of generating the modes themselves.

Résumé: Nanophotonic systems offer unique opportunities for controlling and stimulating light-matter coupling. One of the important topics which drag the interest of the researchers is artificial chirality in quantum sources in interaction with surface photonic or plasmonic modes. The presence of the transverse optical spin component of guided modes opens a possibility to control their direction of propagation by means of the so called spin-locking effect. The atom transition with nonzero spin moment can excite surface modes in preferable direction, or, on the contrary, the scattering of surface mode is very sensitive to chirality of atomic transitions.
In this work we present a brief overview of our recent theoretical results on coupling of chiral atoms with surface guiding modes. In particular, we demonstrate that the scattering of nonfiber guided mode on ensemble of atoms with chiral transitions, and show how the spin of nanofiber modes governs the scattering spectrum. Moreover, we propose a model of atomic ensemble of chiral atoms with perfect unidirectional coupling, and suggest a rigorous solution in such system, which demonstrate the main features of unidirectional coupling. Such system can be implemented with a simple metal nanowire, where the spin-locking effect is extremely strong. Finally, we will discuss the effect of quantum anisotropy, which allows coupling of orthogonal quantum states close to a nanophotonics systems. We show by using anisotropic metasurfaces one can couple two atomic levels with chiral transition, which may results in non-inverse Rabi oscillation between two quantum states in a single atom.

Résumé: Light-matter interfaces play a crucial role in the context of quantum information
networks, enabling for instance the reversible mapping of quantum state of light onto
quantum states of matter. A promising approach for the realization of such interfaces is
based on ensemble of neutral atoms. A critical figure of merit of such interfaces is the
overall storage-and-retrieval efficiency, which is mainly determined by technical losses and
atomic decoherence, and depends on the storage mechanism and matter properties.
Collective and cooperative effects manifistable in an atomic ensemble could provide
essential enhancement of the coupling strength between the light and atomic systems. In this
context, one of the strongest requirements to obtain a high efficiency is a large optical
depth, which can be achieved by increasing the size of the atomic system or atomic density
in the system. Moreover, recent experimental advances in the trapping technique have made
it possible to create 1D, 2D or 3D spatially ordered atomic configurations, where the
collective effects play a very important role. In addition, the interaction between light and
atoms can be enhanced by trapping atoms in the vicinity of a nanoscale waveguide due to
strong confinement of the light.
In this context, in this talk I will discuss light propagation in a spatially dense atomic
ensemble, where the average distance between atoms is comparable with the resonant
wavelength. In such dense atomic configurations dipole-dipole interaction play an important
role and can lead to manifestation of super and subradiance effects. I will consider the light
propagation in both free space and trapped near nanofiber surface atomic ensembles. The
light scattering in such dense atomic configuration is described in terms of microscopic
approach based on the standard scattering matrix and Resolvent operator formalism. We
show theoretically and experimentally that spatially dense atomic ensembles allow
obtaining effective light-matter interface and reliable light storage with essentially fewer
atoms than it can be achieved in dilute gases. Furthermore, we show that the presence of an
optical nanofiber modifies the character of atomic interaction and results in long-range
dipole-dipole coupling between atoms not only via the free space, but also through the
waveguide mode.

Résumé: In this talk, we are interested in a transmission problem between a dielectric and a metamaterial.
The question we consider is the following: does the limiting amplitude principle hold in such a
medium? This principle defines the stationary regime as the large time asymptotic behavior of a
system subject to a periodic excitation.
An answer is proposed here in the case of a two-layered medium composed of a dielectric and
a particular metamaterial (Drude model). In this context, we reformulate the time-dependent
Maxwell’s equations as a conservative Schr¨odinger equation and perform its complete spectral
analysis. This permits a quasi-explicit representation of the solution via the ”generalized diagonalization”
of the associated unbounded self-adjoint operator. As an application of this study,
we show finally that the limiting amplitude principle holds except for a particular fequency, called
the plasmonic frequency, characterized by a ratio of permittivities and permeabilities equal to − 1
across the interface. This frequency is a resonance of the system and the response to this excitation
blows up linearly in time.

Résumé: Bright squeezed vacuum (BSV) is a quantum state of light that can be produced from a strongly pumped unseeded parametric amplifier, its brightness can be comparable with lasers (more than 10 trillion photons per mode). In this seminar I will present two applications of bright squeezed light. First I will report on interferometric measurements beyond the shot noise limit using a so-called SU(1,1) interferometer. I will then describe experiments using bright squeezed light as a pump to generate nonlinear effects. The sensitivity of an interferometric measurement on a phase shift depends on the state of light used as a probe and the measurement scheme. A ‘standard’ precision is provided by a coherent state fed into a Mach-Zender interferometer, the so-called shot noise limit (SNL). A measurement beating this limit is said to be supersensitive. In order to make super-sensitive phase measurements, quantum resources can be used. For example, squeezed light is now implemented in gravitational wave detectors. Besides the input state and the detection scheme, one can also modify the interferometer. Two optical parametric amplifiers (OPAs) can be used instead of the passive beam-splitters of conventional interferometric setups to form an SU(1,1) interferometer, the phase sensitive response of the OPAs giving rise to interference patterns. Using this interferometer, we demonstrate a phase sensitivity overcoming the shot noise limit by 2.3 dB and show the remarkable robustness of this scheme against detection losses.
I will then show that BSV, i.e. amplified quantum fluctuations, turns out to be very useful for pumping multiphoton effects despite the common opinion that large intensity noise is detrimental for any applications. In particular, BSV can be used to enhance the occurence of extreme events and rogue waves in nonlinear optics and to create states with exotic photon statistics.

Résumé: The late time behavior of disordered wave bearing systems, acoustic, elastic or optical, has been of great interest in recent years. From a practical perspective, late time measurements correspond to large distances in a disordered system, and are thus relevant in many imaging or non-destructive
evaluation applications. From a basic science perspective, late time ob-
servations can reveal interference behavior resulting from the approach to
the Anderson transition, and offer the prospect of observing classical wave
systems in a localized phase. While the Anderson transition has been the-
oretically predicted to occur in such systems for some time, clear evidence
of a localized phase has unexpectedly been quite difficult to find. Our theo-
retical understanding of the current status is still incomplete, with continu-
ing efforts employing direct numerical simulations, self-consistent, multiple
scattering models, and field theoretic methods. Of these, direct numerical
simulations and field theoretic methods enable a description of the local-
ized phase. Direct numerical simulations, particularly those based on a
monopole or dipole scattering approximation, have achieved some promis-
ing results but still describe highly idealized systems, and require substan-
tial computational resources to describe systems substantially larger than a
mean free path. Field theoretic methods on the other hand, while yielding
semi-analytic results, have in the past been restricted to even more idealized
systems. We discuss recent results in these areas, particularly with a view of
moving towards a more realistic description of disordered, elastic systems.

Résumé:
Granular matter display both solidlike and fluidlike properties, and are ubiquitous in nature and industrial applications. A variety of phenomena observed in driven granular matter can be attributed to glassy dynamics -- namely, local contact changes and rearrangements at loose spots. In this talk, I present an overview of the Shear-Transformation-Zone (STZ) theory, a statistical description of granular flow and the dynamics of flow heterogeneities in unconsolidated granular matter, consistent with the principles of nonequilibrium thermodynamics. The propensity for flow heterogeneities or "shear transformation zones" to rearrange and produce nonaffine strain is given by a thermodynamically-defined structural effective temperature known as the compactivity. I will discuss applications of the STZ theory in describing stick-slip instabiltiies in granular flow, and resonance shifts in nonlinear acoustic experiments. If time permits, I will also discuss an STZ description of relaxation in a granular glass bead pack.

Résumé: In this study, we establish an inclusive paradigm for the homogenization of scalar wave motion in periodic media (with or without the source term) at finite frequencies and wavelengths spanning the first Brioullin zone. We take the eigenvalue problem for the unit cell of periodicity as a point of departure, and we consider the projection of germane Bloch wave function onto a suitable eigenfunction as descriptor of effective wave motion. For generality the finite wavenumber, finite frequency (FW-FF) homogenization is pursued in Rd via second-order asymptotic expansion about the apexes of “wavenumber quadrants” comprising the first Brioullin zone, at frequencies near given (optical) dispersion branch. We also consider the degenerate situations of crossing or merging dispersion branches with arbitrary multiplicity, where the effective description of wave motion reveals several distinct asymptotic regimes depending on the symmetries of the eigenfunction basis affiliated with a repeated eigenvalue. One of these regimes – for whose occurrence we expose a sufficient condition – is shown to describe the so-called Dirac points, i.e. conical contacts between dispersion surfaces, that are relevant to the phenomenon of topological insulation. For all cases considered, the effective description turns out to admit the same general framework, with differences largely being limited to (i) the basis eigenfunction, (ii) the reference cell of medium periodicity, and (iii) the wavenumber-frequency scaling law underpinning the asymptotic expansion. We illustrate the utility of our analysis by several examples, including an asymptotic description of the Green’s function near the edge of a band gap.

Résumé: Semi-conductor lasers invented in 1962 are vital to our modern daily life. For example, they generate the optical impulses that carry ever-greater amounts of information in fiber-optic networks over great distances. The emergence of irregular and unpredictable pulsations and dynamical instabilities from a laser were first noted during the very early stages of the development of lasers. Pulses with amplitude varying in an erratic manner were reported in the output of the ruby solid-state laser. However, the lack of knowledge of what would later be termed butterfly effect i.e. deterministic chaos resulted in these initial observations being either left unexplained or wrongly attributed to noise. This presentation will highlight the fundamental physics underpinning the butterfly effect in semiconductor lasers and also the opportunities in harnessing it for potential applications.

Résumé: Granular media exhibit significant dependence on its flow history. This behaviour has, however, not been sufficiently studied or considered in constitutive models. In this talk, I will present numerical results from discrete element simulations of dry granular materials and granular suspensions under simple shear flow. Unsteady flow is introduced through a reversal of shear direction after the flow reaches steady state. The bulk stresses display striking evolution following reversal at a strain scale of unity or larger. This large-scale evolution is common to both dry and wet flows, and related to the re-orientation of the anisotropic microstructure. When hydrodynamic interactions are included, however, a dramatic response in hydrodynamic stress occurs at very a small strain scale, which is controlled by the non-hydrodynamic surface interactions. We further established a constitutive model to describe such unsteady flows in the rate-independent regime. The model consists of a stress equation and an evolution equation of a second-order fabric tensor, the second invariant of which is used to characterise the microstructure anisotropy and the trace is the coordination number. The pressure and a scalar viscosity are modelled as functions of the fabric tensor. Even with this relatively basic incorporation of fabric (without resorting to a fourth-order viscosity tensor), the model is shown to be able to capture well the stress evolution in shear reversal and predict more complex flows under cyclic loading.

Résumé: Multiple diffusion resulting from the propagation of waves in disordered environments, such as light passing through biological tissues or a fine layer of paint, is an extremely complex process but remains linear. In fact, with spatially discrete inputs and outputs, it can perform the equivalent of a random projection, i.e. the multiplication of the input vector by an iid random matrix. In this context, we have shown that these environments act precisely as "compressed sensing" model systems, allowing signal acquisition with a number of measurements driven by the actual amount of information. Conversely, one can see this physical system as an optimal mixer of information, performing instantaneously in the (physical) analog domain an elementary computation brick of many Machine Learning schemes. We will present a series of proof of concept experiments in Machine Learning, and discuss recent technological developments of optical co-processors within the startup LightOn, co-founded with Igor Carron, Sylvain Gigan & Florent Krzakala.

Résumé: Recently, photoacoustic microscopy (PAM) has attracted
attention to visualize deep structures in living tissues. However, it is
difficult to improve the spatial resolution of PAM without using
high-frequency components of photoacoustic waves, which are not suitable
for deep imaging. To overcome this drawback, we have developed
two-photon absorption-induced photoacoustic microscopy (TP-PAM). The
spatial resolution in TP-PAM is determined by two-photon absorption
(TPA). The use of low-frequency ultrasonic components generated by TPA
enables PAM to visualize deeper structures while preserving the high
spatial resolution.

Résumé: Helicopters face huge risks when landing or taking-off in arid area: the rotor airflow stirs up the sand grains and create sand clouds, called brownout, that can annul the pilot's visibility. We study a solution to avoid accidents that may occur: an active THz imaging system to image through the brownout. My work is composed of two parts. On the one hand, we offer an analytical model to evaluate the performance of the imaging system. On the other hand, we evaluate experimentally the propagation of THz radiation through sand clouds.

Résumé: Gaps formed between metal surfaces control the coupling of localized plasmons, thus allowing gap-tuning targeted to exploit the enhanced optical fields for different applications. Classical electrodynamics fails to describe this coupling across sub-nm gaps, where quantum effects
become important owing to non-local screening and spill-out of electrons. The advantages of narrow gap antennas have mostly been demonstrated for processes like SERS that are excited optically, but promising new phenomena appear when such antennas are fed by electric generators. However, the extreme difficulty of engineering and probing an electrically driven optical nanogap antenna has limited experimental investigations of physical concepts at stake in these conditions. The feasibility of structuring electron-fed antennas as nano-light sources has been recently demonstrated; however, the suggested configuration remains very limited: too much power was lost as heat when operating the optical antenna, and the antenna operation time was limited by the structure lifetime to sustain a bias voltage for a few hours. The innovative structure that I propose in my talk will cope with all these limitations: ALD dielectric materials substitute the air
gap to improve the antenna stability; a quantum efficiency of 0.1 is targeted owing to a significantly efficient antenna (2 orders of magnitude higher field enhancement). The resulting source will operate at room temperature and have a tunable spectral response (ranging from
visible frequencies to THz regime) defined by the antenna geometry and the applied bias. Also, this source will be compact, Si-compatible, and will not request specific emitting materials (e.g. III-V semi-conductors) to operate.

Résumé: When confining photons in semiconductor lattices, it is possible deeply modifying their physical properties. Photons can behave as finite or even infinite mass particles, photons can propagate along edge states without back scattering, photons can become superfluid, photons can behave as interacting particles. These are just a few examples of properties that can be imprinted into fluids of light in semiconductor lattices. Such manipulation of light present not only potential for applications in photonics, but great promise for fundamental studies. One can invent artificial media with very exotic physical properties at the single particle level or even more interestingly when many body interactions are considered. During the talk, I will illustrate the variety of physical systems we can emulate with fluids of light by presenting a few recent experiments. Perspectives in terms of quantum simulation will be discussed.

Résumé: In this talk I present two different systems that are built up of resonantly coupled subwavelength particles. The first system is a disordered cloud of cold atoms, whereas the second system is a hexagonal array of silicon nanopillars. In both cases the individual entities are resonantly coupled to each other, which leads to an interaction with light that is different than the sum of individual particles. In a wavelength-sized cloud of cold atoms we theoretically show that this disordered system actually shows order in its optical response by the presence of some particular collective modes. We also predict a special regime of light scattering from dense, subwavelength-sized clouds of atoms. The ensemble of atoms scatters less light than a single atom does, precisely due to their strong near-field interactions. In the second part of my talk, I focus on hexagonal lattices of silicon nanopillars. These arrays show collective modes which we can this time experimentally visualize with 10 nm resolution thanks to cathodoluminescence spectroscopy. With this technique, we focus a 30-keV electron beam on the sample. The electrons have a strong electric field surrounding them and serve as a localized coherent broadband optical excitation source that excites modes inside the nanopillars at the nanometer scale. The periodic repetition of a resonant nanopillar leads to the creation of a band diagram; it is a photonic crystal slab. With angle-resolved cathodoluminescence spectroscopy we have access to that band diagram in a single measurement. We found experimental evidence of the existence of modes with an effective refractive index very close to one.

Résumé: Light shaping is an established division of modern optics, at the origin of many applications for communication, computing and imaging. In this work, we generalize light shaping to the quantum domain. We show that patterns of phase modulation for classical laser light can also shape higher orders of spatial coherence, allowing deterministic tailoring of high-dimensional entanglement. By modulating spatially entangled photon pairs, we create periodic, topological, and random structures of quantum illumination, without effect on intensity. We then structure the quantum illumination to simultaneously compensate for entanglement that has been randomized by a scattering medium and to characterize the medium's properties via a quantum measurement of the optical memory effect. The results demonstrate fundamental aspects of spatial coherence and open the field of adaptive quantum optics.

Résumé: Optical coherence tomography (OCT) enables volumetric rendering and the generation of fundus images that precisely register OCT images to fundus features. Yet, there is little data about how a child’s retina develops. This limits our knowledge of how diseases affect a child’s vision early in life and makes diagnosis of these diseases more difficult. The introduction of OCT to pediatric applications has been impeded by several factors, among them limited speed of data acquisition and analysis, difficulty in attaining stable fixation of the pediatric patient on a target over a period of time long enough to allow reliable analysis, short working distance, etc. The system described here integrates three major components: a) a computer-controlled video player that plays attention attracting movies and directs the subject’s fixation to a central point target, b) a retinal birefringence scanning (RBS) subsystem for fast detection of central fixation by detecting the position of the fovea, and c) a long-working-distance optical coherence tomography subsystem for acquiring 3D images from the retina. Significant issues need to be resolved. These include separating the two systems spectrally, presenting suitable visual targets on a small LCD screen, communication between the two systems in real time, precise alignment, simultaneous aiming of the two systems, etc. Of particular importance is the fact that most optical components in the combined path can also affect the polarization state of light in the RBS path, as does the human cornea. To address this issue, a computer model was employed, to optimize system performance. The hybrid system integrating OCT and RBS acquires and analyzes data only during moments of central fixation. This is expected to significantly reduce the image processing time and shorten overall exam duration.

Résumé: The presentation will start with a brief introduction to the Radiative Transfer Equation (RTE) and its numerical solution with the Monte Carlo method. Firstly, applications of the RTE to the Earth's atmosphere will be considered, in particular for the detection of small non-spherical ice particles in optically thin cirrus. Then, in the context of biomedical optics, the RTE is applied to model light propagation in biological tissues. Using the reciprocity relation for the vector RTE, the benefits of the use of polarized light are demonstrated for diffuse optical tomography. These results are extended through a perturbative approach to consider temporal correlations in media with moving particles undergoing Brownian motion. As a result, applications of the RTE to diffuse correlation tomography and speckle contrast optical tomography are presented. In the last part, the RTE is used to simulate the coherent wave nature of the electromagnetic field. In particular, the propagation of partially coherent beams through random particulate media is considered.

Résumé: Recently, mathematical modeling of cancer started drawing great interest from the medical community. Indeed, developing models able to describe accurately tumor growth may help monitoring the disease evolution or even predicting the efficacy of different therapeutic strategies. Such applications are made possible through the routine monitoring of patients with imaging devices. This offers a consistent amount of valuable data to elaborate and validate the mathematical models. The aim of my talk is to present a quick overview of the strategies that we have recently developed in our team at Bordeaux.
In a first part, I will quickly present a simple tumor growth model based on a mechanistic description of the healthy and tumor cell densities evolution over time. This model is valid for example for meningiomas or for some lung metastases, i.e. when there is no treatment (only the growth is considered) and when the shape is approximatively the same over time. This model presents both the interest to be parametrizable - using the tumor volumes at 2 times and the initial shape tumor - for each considered patient and to produce reliable predictions and 3D extension simulation within a reasonable computing timescale. The parameters estimation strategy based on a reduced 0D-model and on stochastic methods (Monte-Carlo-like methods) will be presented.
When we consider treatments (tumour decay) or/and time-evolving shapes, more complex models - with different tumour cell densities - have to be written and more information - issued from medical imaging - is necessary to parametrize them. In a second part of my talk, I will introduce these complex models and I will present the information that can be extracted from medical imaging such as textures or shapes of the lesions. I will show why the parameter estimation approach used for the simple model is not anymore available and I will propose a new strategy based on data assimilation strategy. I will present a Luenberger - also called nudging - state observer coupled with a parameter Kalman-based observer to perform a joint state and parameter estimation. I will illustrate my results with synthetic and real data.

Résumé: This contribution will start with a brief introduction to nanophotonics and plasmonics. In this context, different
nanofabrication techniques will be discussed with an emphasis on the DNA-Origami approach. In particular, we employ this technique as a platform where metallic nanoparticles as well as single organic fluorophores can be organized with nanometer precision in three dimensions. With these hybrid structures we initially study the nanoparticle-fluorophore interaction in terms of the distance-dependent fluorescence quenching and angular dependence around the nanoparticle. Based on these findings, we build highly efficient nano-antennas based on 100 nm gold dimers which are able to strongly focus light into the sub-wavelength region where the fluorophore is positioned and produce a fluorescence enhancement of more than three orders of magnitude. Using this highly confined excitation field we were able to perform single molecule measurements in solution at concentrations as high as 25 µM in the biologically relevant range ( larger than 1µM). Additionally, we report on a controlled increment of the radiative rate of organic dyes in the vicinity of gold nanoparticles with the consequent increment in the number of total emitted photons. We also employ the nanoantennas to mediate the fluorophore emission and thus to shift the apparent emission origin: A single molecule mirage. We will discuss how DNA-Origami can also improve the occupation of other photonic structures, the zeromode waveguides (ZMWs). These structures, which consist of small holes in aluminum films can serve as ultra-small observation volumes for single-molecule spectroscopy at high, biologically relevant concentrations and are commercially used for real-time DNA sequencing. To benefit from the single-molecule approach, each ZMW should be filled with one target molecule which is not possible with stochastic immobilization schemes by adapting the concentration and incubation time. We present DNA origami nano-adapters that by size exclusion allow placing of exactly one molecule per ZMW. The DNA origami nano-adapters thus overcome Poissonian statistics of molecule positioning and furthermore improve the photophysical homogeneity of the immobilized fluorescent dyes. Finally, we will discuss future potential applications of this technology on smartphone-based point of care diagnostic platforms and diagnostics.

Résumé: The development of micro‐ and nanotechnologies has recently opened a wide range of possibilities for controlling light at the wavelength scale or below. A fine control of the light (emission, transport and detection) in small volumes is at the heart of various applications, such as high performance sensors, light focusing below the diffraction limit, nanolasers, solid‐state single‐photons sources, or photovoltaic devices. Most of these applications rely on the use of optical resonances, which can be either localized in small volumes or delocalized over the system.
Plasmonic nanoantennas (or nanoresonators) are able to confine light in extremely small volume of the order of 1/10,000 of a cubic wavelength, i.e., well below the diffraction limit. Light confinement is associated with large field exaltations that can be used to enhance non‐linearities, spontaneous emission, or absorption. In the context of spontaneous emission, these properties lead to huge enhancement factors. Unfortunately, very large enhancement factors usually correspond to emitters located only a few nanometers away from a metal surface. As a consequence, quenching through non‐radiative energy transfer to the nearby metal constitutes the dominant relaxation process. The resulting radiative efficiency is extremely low and it is thus often thought that very large spontaneous decay rates in plasmonic nanoantennas are not of practical interest. However, recent experimental demonstrations have shown that it is possible to obtain large enhancement factors (typically above 1000) with non‐negligible radiative efficiencies (a few tenths of percent).
We have developed an electromagnetic theory that is able to unravel the interplay between the decay into the antenna mode and the quenching. The theory that is valid for any plasmonic nanoantenna relies on a modal formalism recently developed for photonic and plasmonic nanoresonators. In particular, we have shown that overcoming the quenching is possible in different types of metallo‐dielectric nanoantennas. Such an in‐depth quantitative analysis has allowed us to provide guidelines to design plasmonic nanoantennas that are able to overcome quenching and provide both large spontaneous decay rates and large radiative efficiencies. We have also studied the problem of absorption enhancement by a plasmonic nanoantenna and derived meaningful figures of merit. The latter provide an important insight into the limits of absorption enhancement.

Résumé: Nanoparticles have significant advantages for biomedical applications, especially for the vectorization (possibly traceable) of active biomolecules, for labeling, or even as probes of some physicochemical parameters at the nanoscale.
The diamond nanocrystal (nanodiamond) is a remarkable system with intrinsic properties allowing all these applications simultaneously. We will show that nanodiamonds rendered fluorescent by the creation of embedded color centers, can be used to measure the intraneuronal transport parameters in cultured neurons.
Thanks to the high brightness and the excellent stability of nanodiamond photoluminescence we were able to correlate intraneuronal transport abnormalities to subtle genetic alterations found in neuropsychiatric disorders, and modeled in transgenic mices. Such an approach could provide an unbiased diagnosis of these complex diseases.
In an other biomedical application, still in cultured cells, we used nanodiamond coated with cationic polymer to transport therapeutic oligonucleotides (siRNA) against the sequence of a junction oncogene responsible for Ewing sarcoma (a rare bone cancer). Compared to a conventional approach, the vectorization of the interfering RNA by the nanodiamond lead to a stronger inhibition of the oncogene responsible of cell proliferation.
We will conclude by in vivo prospects of these two applications (intraneuronal transport and nanomedicine), introducing an alternative nanolabel, the silicon carbide nanocrystal, that offers near-infrared responses (fluorescence and second-harmonic generation), in the transparency spectral range of tissues.

Résumé: Single-photon sources are crucial to a number of applications that take advantage of the exotic phenomena stemming from the laws of quantum physics. Color centers in diamond have gained much attention in this context, essentially for their unique optical properties at room temperature, but a substantial basic research effort in nanophotonics and electronics is still required.
In my seminar I will introduce nano-optical concepts to largely improve single-photon sources and the work that we have recently undertaken towards obtaining highly efficient single-photon sources based on the silicon-vacancy color center in diamond. I will present results pertaining to diamond implantation and characterization as well as on the design of antenna configurations that can significantly improve the outcoupling efficiency and the emission rate. I will also discuss some ideas on the possibility of electrical pumping and the expected performances.

Résumé: Topology optimization is one of structural optimization methods that
optimizes material layout within a given design domain for a given set of
loads, boundary conditions and constraint maximizing the performance of the
system. Currently, engineers use topology optimization at the conceptual
level in design process. In this talk, the application of topology
optimization for multiphysics system will be given. The multiphysics systems
such as acoustic-structure interaction, fluid-structure interaction,
fluid-thermal and porous-acoustic coupling are hard to design by the
imagination of engineers. Therefore, topology optimization method plays an
important role. In the applications, the conventional analysis theories are
hard to apply because not only material properties but also governing
equations should be interpolated. Therefore some novel analysis approaches
should be developed. The author has researched on these subjects for the
last several years. This presentation introduces the concept of topology
optimization and shows several works in multiphysics system. Furthermore, a
new research towards metasurface and metamaterials is presented.

Résumé: From sand dunes to Faraday crispations, granular materials, i.e., large agglomeration of macroscopic particles, are ubiquitous in nature, industry and our daily lives with widespread applications ranging from the prediction of natural disasters (e.g. snow avalanches and debris flows) through the enhancement of energy efficiency in industries (e.g. mining, civil engineering) to emerging new technologies (e.g. powder based additive manufacturing). Due to the energy dissipation at the individual particle level, granular systems are highly dissipative and consequently their stationary states are typically far from thermodynamic equilibrium. Therefore, understanding how the mobility of individual particles influences the collective behavior is crucial in describing granular materials as a continuum.
At the `microscopic' level of individual particles, I will introduce a recently developed microwave radar system that is capable of tracking a metallic particle continuously in three dimensions and discuss its advantages and limitations in comparison to other particle imaging approaches. At the `macroscopic' level of collective motion, I will talk about the pattern forming scenario of partially wet granular materials (e.g., wet sand on the beach) with a focus on how liquid mediated particle-particle interactions influence the collective behavior. In particularly, I will focus on the formation of density-wave fronts in an oscillated wet granular layer undergoing a gas-liquid-like transition and discuss how the emerging time and length scales are associated with the competition between the time scale for the collapse of particles due to short ranged attractive interactions and that of the energy injection resisting this process.

Résumé: In this seminar I will present some of the basic ideas of networks’ theory and their areas of current application in different fields of science, with emphasis in the new field of complex networks. I will present some of main theoretical models of networks and of network formation and , and I will discuss some applications to robustness and resilience, and to spreading and diffusion of processes in networks.

Résumé: The current trend towards compact, cost-effective and multi-purpose infrared opto-electronic systems brings the need for new conception tools and technological means. In this frame, subwavelength and plasmonic concepts open promising avenues: at a first level, for the conception of high-efficiency and compact optical elements arrays (i.e., polarizer, filter, or lens arrays) that can be brought in the vicinity of focal plane arrays, inside the confined volume of the camera; at a second level, for the integration of optical functions within the pixel of detection; and at a third level, for the enhancement of the opto-electronic properties of the elementary infrared detector. I will draw an overview of recent advances and realizations done in our lab.

Résumé: A central challenge in understanding extreme events in physics is to develop rigorous models linking the complex generation dynamics and the associated statistical behaviour. Quantitative studies of extreme phenomena, however, are often hampered in two ways: (i) the intrinsic scarcity of the events under study and (ii) the fact that such events often appear in environments where measurements are difficult. A particular case of interest concerns the infamous oceanic rogue or freak waves that have been associated with many catastrophic maritime disasters. Studying rogue waves under controlled conditions is problematic, and the phenomenon remains a subject of intensive research. On the other hand, there are many qualitative and quantitative links between wave propagation in optics and in hydrodynamics, because a nonlinearly-induced refractive index perturbation to an optical material behaves like a moving fluid and is described mathematically by the same propagation equation as nonlinear waves on deep water. In this context, significant experiments have been reported in optics over the last two years, where advanced measurement techniques have been used to quantify the appearance of extreme localised optical fields that have been termed "optical rogue waves". The analogy between the appearance of localized structures in optics and the rogue waves on the ocean’s surface is both intriguing and attractive, as it opens up possibilities to explore the extreme value dynamics in a convenient benchtop optical environment. The purpose of this talk will be to discuss these results that have been obtained in optics, and to consider both the similarities and the differences with oceanic rogue wave counterparts.

Résumé: The purpose of this talk is to introduce to a broad audience a few
applications of antenna concepts for the manipulation of light.
In the optical range, surface modes called surface plasmon polaritons take
place in the vicinity of metallic antennas, enabling a strong light/matter
interaction within highly confined volumes. In order to take advantage
of this property, three applications of plasmonic antennas will be
investigated.
First, in the case of single-photon sources, both theoretical and
experimental studies of single-emitters performance when coupled to a
planar metallic antenna will be presented. A strategy to enhance its
performance will be studied theoretically.
Then, in the case of electrical generation of light by inelastic
electron tunneling, we will analyse the modification of radiation
properties close to a metallic nano-rod. This analysis paves the way
towards the design of integrated, compact electrical sources of surface
plasmons.
Finally, in the case of detecting a weak quantity of molecules, the
interaction between an infrared light beam and a sub-nanometric layer of
resonant molecules deposited on a nanostructured metallic mirror will be
studied.

Résumé: Modern dense seismological deployments consisting of many hundreds of sensors allow the reconstruction of the seismic wavefield in the near-field from noise cross-correlations. The correlation approach makes it thus feasible to resolve the focus or focal spot, which is a characteristic feature of the cross-correlation wavefield at zero lag time. Based on the equivalence of time-reversal and cross-correlation, applications in acoustics, nondestructive testing, and medical imaging have long been relying on focal spot properties. I introduce focal spot based imaging in seismology on two (very) different scales using data from two dense arrays, each consisting of 1000 stations, that cover a ~1 km2 area and half of the United States, respectively. In the presentation I will provide a brief history of re-focusing in seismology, before discussing aspects of focal spot based imaging that arise in the seismological surface wave context, and that have not been at the center of research in acoustics or elastography. These aspects include the relation to Aki's 1957 spatial autocorrelation method; the shape of the surface wave focal spot depending on horizontal and vertical motion; interference of surface wave fields and body wave fields, effects on the focal spot, and strategies for wave field separation; and the resolution of anisotropy. I will emphasize implications for spatial resolution with an outlook to a transfer of the super-resolution concept to seismology.

Résumé: Anderson localization can be described as the inhibition of wave propagation due to strong disorder. For three-dimensional (3D) systems, there exists a true phase transition between the localized and diffuse regimes. For an electronic system this transition occurs when the energy distribution of sites reaches a critical disorder, or equivalently when electron energy is above some critical value for a fixed disorder strength. In contrast, localization of classical waves in 3D can only occur in a finite band of energy (or frequency), leading to a so-called a mobility gap: a localization regime bounded by a lower and upper transition towards diffuse behaviour. I will present an experimental investigation of a complete mobility gap in 3D, using observations of the dynamic coherent backscattering effect for ultrasonic waves. This method is free of both absorption and any non-linear effects, thereby side-stepping two of the major historical roadblocks in the study of localization, and in some cases can present an advantage over analogous transmission measurements. We fit our data with the self-consistent theory of localization, which has successfully described localized ultrasound in past transmission experiments. Through this technique, we were able to measure the localization length as a function of frequency all the way through the mobility gap.

Résumé: The eye is the only optical window to a neuro-vascular network in our body, located in the retina. Probing this retinal network at the scale of optical wavelengths (typically a micron) provides insight not only on the major eye diseases (Age-related Macular Degeneration, Glaucoma, Diabetic retinopathy), but also on the state of the brain neuro-vascular network and on the related pathologies (Alzheimer, Parkinson, Traumatic brain injuries).
These developments have been triggered by the adaptation of adaptive optics to ophthalmology since 1997. I started to work on the subject more than 10 years after. Before building yet another adaptive optics ophthalmoscope, I decided to investigate the optical properties of the eye, and of the aberrations the adaptive optics was to correct for. Indeed, unlike in astronomy, no reliable statistical model of eye motion and aberration dynamics was available.
I will present the results of an aberrometry campaign on 50 eyes, and discuss the link between motion and aberration. Then, I will show the latest retinal imaging results we obtained with the ECUROeil adaptive optics ophthalmoscope, a system we built and designed based on the aberrometry results. These images reveal blood flow in arteries and capilaries at 200Hz, as well as the fine structure of the optic nerve head. Some of these images prompted us to design novel imaging instruments, using high resolution structured illumination.

Résumé: Hydroelastic waves appear at very large scale in the marginal ice zone where the ocean is covered by a thin layer of ice. The physics of these surface waves is dominated by the bending elasticity of the thin sheet covering the liquid, but their propagation is much slower than plate (Lamb) waves thanks to the fluid inertia. In this seminar, I will describe our experimental approach of hydroelastic waves. I will first focus on the typical parameter range in which they can be observed at the laboratory scale. In a second step I will show that these waves open promising possibilities for wave control. In particular, I will present experimental configurations that allow for building hydroelastic waves based "optics", revisiting Snell's law, geometrical optics and Fourier optics in an hydrodynamics experiment. We also investigate the properties of resonators for our system, created using simple perforations in our elastic cover.

Résumé: Ultrasonic nondestructive evaluation (NDE) is an emerging technology that enables to raise the remaining life and reliability of nowadays structures, as well as to characterize pathologies in medical science. Typically, four levels of NDE are considered: (1) the detection of the presence of a damage, (2) the localization of that damage, (3) the identification and quantification of that damage, and (4) its influence on the remaining life of the structure. The concept of damage/pathology is here understood in a broad sense, which ranges from defects in a structural material to consistency changes in a biological materials.
For competitive damage assessment and quality control, quantitative NDE techniques based on the use of theoretical models of the ultrasonic wave propagation have been developed to extract additional information from experimental measurements. Despite the structural complexity of the considered materials (e.g., spatial heterogeneity of the mechanical properties, multiple damage mechanisms, dispersion, porosity, attenuation), relative simple models are required for efficient and real-time monitoring of the structure. Indeed, the complexity of the recorded signals suggests to directly compare the experimental measurements with theoretical results, with the purpose of extracting quantitative information from damage or pathology. A possible approach to solve this problem is provided by the model-based inverse problem (IP) framework. The solution of an IP identification approach is commonly defined in terms of the minimization of an objective function consisting in the discrepancy between the experimental observations and the numerically predicted results.
The purpose of this talk is to present three potential applications of such model-based inverse problems, namely (1) the characterization of CFRP plates subjected to post-impact fatigue damage using an ultrasonic through-transmission setup; (2) cortical bone assessment using ultrasonic guided waves; and (3) the investigation of interfacial stiffnesses of a tri-layer using zero-group velocity Lamb modes.

Résumé: In disordered media, the absence of diffusion arising from the spatial localization of single-particle states is known as Anderson localisation (AL). In three dimensions, AL manifests itself as a phase transition which occurs at a critical energy or at a critical disorder strength (the mobility edge) separating a metallic phase where states are spatially extended, from an insulating one where states are localized. Theoretically, much efforts have been devoted to the study of the critical properties of the Anderson transition (AT), such as wave-function multifractality or critical exponents. In practice however, only a handful of experiments have found evidence for the 3D Anderson transition, among them cold atoms, and even fewer have investigated its critical features (mostly in the context of quantum-chaotic dynamical localization). In addition to the intrinsic difficulty of achieving wave localization in three dimensions, one reason for the rareness of experimental characterizations of the Anderson transition is the lack of easily measurable observables displaying criticality. In this talk, I will show that the critical properties of the AT are encoded in two emblematic interference effects observed in momentum space: the coherent backscattering (CBS) and the coherent forward scattering (CFS) peaks, the latter being an effective ``order parameter’’ of the transition. By a finite time scaling analysis of the CBS width and of the CFS contrast temporal dynamics, one can extract accurate values of the mobility edge and critical exponents of the transition in agreement with their best known values to this date.

Résumé: Wavefront shaping is the capability to control the incident light towards optimal coupling with a complex system. Applying wavefront shaping in Nanohotonics allowed us to focus light in the nanoscale and the capability to achieve super-resolution imaging. Similarly, we could focus light in space and time at the location of target particles embedded in random scattering media. The outlook for both scenarios is to deliver highly concentrated optical energy in a local and controllable way so as to address with high sigbnal-to-noise ration relevant problems in biomedical imaging, sensing and therapy.

Résumé: During this seminar, I will show you that Michelson interferometers ae not only complicated experiments to scare students during their bachelor, but can also be of crucial importance for some researchers, as they enable the detection of sub-wavelength mechanical vibrations, even “deep” inside biological tissues. I will present our development of different optical microscopes, based on interferometry that can simultaneously measure some mechanical and biochemical information in various biological samples. We could notably demonstrate that subcellular mechanical fluctuations depend on cellular metabolic activity, offering us an original contrast to detect cells and their pathologies. Finally, I will also present our attempt to detect an electromechanical coupling in mammalian neurons with our microscopes, in order to demonstrate more complete theories explaining how information propagates in the brain.

Résumé: The aim of this talk is to give a pedagogical introduction to light scattering from random and ordered rough surface. This will be done by presenting a collection ("a potpourri") of rough surface scattering phenomena. We will discuss what characterize them, under what conditions they occur, and what is their physical origin. Some of the phenomena we will present are enhanced back and forward scattering; satellite peaks; the Brewster scattering phenomenon; optical Yoneda peaks; Rayleigh and Wood anomalies; and Mueller matrices and depolarization.

Résumé: Many optical techniques have been developed for real-time imaging of blood flow. Laser speckle contrast imaging has become one of the most widely used techniques due to its simple instrumentation and its ability to visualize blood flow over a wide range of spatial scales. However, obtaining quantitative blood flow information remains a challenge for laser speckle imaging. Recently, an extension to laser speckle imaging, called Multi-Exposure Speckle Imaging (MESI), was introduced that increases the quantitative accuracy of CBF images. This talk will describe technical developments in laser speckle imaging as well as new methods for three-dimensional visualization of blood vessels that can be used to improve our understanding of blood flow measures inferred from speckle images.

Granular friction: from building the pyramids to the anatomy of individual contacts at the nanoscaleDaniel Bonn (van der Waals-Zeeman Institute, University of Amsterdam)mardi 11 octobre 2016, 11:00, Salle 310

Résumé: I will discuss the rheology and mechanical properties of wet granular materials, and show why the behavior can be very subtle. Once one understands the mechanical properties, I will show that one can use this knowledge to construct the perfect sandcastle, or to understand why the ancient Egyptians wetted the desert sand with water before sliding heavy stones over it.
I will then go on to show some new results on friction at the microscopic scale, between 2 grains. Amonton’s famous friction law states that the friction force is proportional to the normal force since both are proportional to the area of contact. However for spherical grains, the contact area is not proportional to the normal force, as shown by Hertz long ago. We use a new fluorescence technique that allows us to probe the real area of contact between 2 rough surfaces. In our case, we conclude that important deviations from Amonton’s law are observed.

Résumé: I will present a first principles analysis of the electromagnetic response of homogeneous and isotropic media.
Using such analysis I will show that space-time causality defines necessary and sufficient conditions for negative refraction. In particular no real stable media can support negative refraction in absence of spatial non-locality and
dissipation, while a generic first order correction to local dissipative response is sufficient to have negative
refraction. Such results provide a sound answer to the long-standing discussion on the conditions for negative refraction, and they open the way to the classification of media with negative refraction. We see many possible extensions and applications of our anlysis to other domains: such as non-homogeneus or non - isotropic material, surface waves, sound waves, etc. with both theoretical and experimental implications.
We look forward to discuss together possible applications of such formalism and results.

Résumé: Random mode mixing in multimode optical fibers has long been
considered a nuisance that should be circumvented. However, it was
recently realized that fibers with strong mode mixing provide
outstanding opportunities for studying the transport of light in
disordered media, as they exhibit two unique advantages over
scattering samples. First, the transmission through fibers is
extremely high, even in the presence of strong mode mixing, and thus
information of the input state of the light is only scrambled but not
lost. Second, unlike random scattering samples, fibers allow to fully
control the coupling of the input light to all the guided modes,
thanks to their finite numerical aperture. We have recently used these
properties of multimode fibers to study new features of coherent
backscattering, also known as weak localization of light. By utilizing
a magneto-optical effect, we controlled the interference between
time-reversed paths inside a multimode fiber with strong mode mixing,
observed for the first time the optical analogue of weak
anti-localization, and realized a continuous transition from weak
localization to anti-localization [1].
Surprisingly, the chaotic-like dynamics of light in multimode fiber
and the extreme sensitivity to external perturbations, open the door
for new types of applications. We have recently shown that multimode
fibers can be used to generate and distribute secure keys for optical
encryption [2]. The fast fluctuations in the fiber mode mixing provide
the source of randomness for the key generation, and the optical
reciprocity principle guarantees that the keys at the two ends of the
fiber are identical. We experimentally demonstrated the scheme using
classical light and off-the-shelf components, paving the way towards
cost‑effective key establishment at the physical-layer of fiber-optic
networks.
[1] Y. Bromberg, B. Redding, S. M. Popoff, and H. Cao, Control of
coherent backscattering by breaking optical reciprocity, Phys. Rev. A
93, 023826 (2016)
[2] Y. Bromberg, B. Redding, S. M. Popoff, and H. Cao, Remote key
establishment by mode mixing in multimode fibers and optical
reciprocity, arXiv:1506.07892

Résumé: Biological tissues are very strong light-scattering media. As a consequence, current medical imaging devices do not allow deep optical imaging unless invasive techniques are used. Acousto-optic (AO) imaging is a light-ultrasound coupling technique that takes advantage of the ballistic propagation of ultrasound in biological tissues to access optical contrast with a millimeter resolution. Coupled to commercial ultrasound (US) scanners, it could add useful information to increase US specificity. Thanks to photorefractive crystals, we developed a bimodal AO/US imaging setup based on wavefront adaptive holography that recently showed promising ex vivo results on tumors for which acoustical contrast were not significant. However, before any clinical applications can be thought of, two major issues of in vivo imaging have to be addressed.
The first one concerns current AO sequences that take several tens of seconds to form an image, far too slow for clinical imaging. The second issue concerns in vivo speckle decorrelation that occurs over less than 1 ms, too fast for photorefractive crystals. In this talk, I will present a new US sequence that allows increasing the framerate of at least one order of magnitude and an alternative light detection scheme based on spectral holeburning in rare-earth doped crystals that allows beating speckle decorrelation as first steps toward in vivo imaging.

Résumé:
What would be the equivalent of looking into a glass mirror in time? Might we go through such a looking glass? These questions which seem to originate directly from a character in Alice in Wonderland entail fundamental interrogations about time reversible systems. Time and space play a similar role in (whatever) wave propagation (optical, acoustical, matter, water, ... waves). Wave control is usually performed by spatially engineering the properties of a medium but temporally manipulating these properties constitute a complementary approach. We demonstrate the relevance of this approach by introducing the concept of Instantaneous Time Mirror (ITM). When a wave propagates in a medium which undergoes at once a strong and brief change of its effective propagation properties, a time reversed, back-propagating wave instantly generated. The effect -in the time domain- of this disruption on the initial wave is the same as the effect in space of a (standard) mirror.
The ITM concept is general and may be applied to any type of wave. We validated this new instantaneous method of time reversal experimentally with water waves. A disruption is obtained by "shaking" abruptly the liquid in order to modify the effective gravity. This experiment is a first realization of the Gedankenexperiment imagined by Loschmidt in a famous dispute with Boltzmann about the time reversibility of the dynamics of gases.
The principle of the ITM will be extended to a periodic time modulation of the medium. The waves existing in this time crystal (or time Bragg mirror) feature peculiar properties. The similarities and differences with their spatial counterpart will be addressed and we will show that it allows us to bring together in a single general concept phenomena as apparently different as phase conjugation (in optics) and the hydrodynamic Faraday instability.

Résumé: There is a growing interest nowadays in the study of strong light-matter interaction at the nanoscale, specifically between plasmons and emitters. Researchers in the fields of plasmonics, nanooptics and nanophotonics are constantly exploring new ways to control and enhance surface plasmon launching, propagation, and localization. Moreover, emitters placed in the vicinity of metallic nanoantennas exhibit a fluorescence rate enhancement due to the increase in the electromagnetic field confinement. However, numerous applications such as optical electronics, nanofabrication and sensing devices require a very high optical resolution which is limited by the diffraction limit.
Targeting this problem, we introduce a novel plasmonic structure consisting of nanoantennas integrated in the center of ring diffraction gratings. Propagating surface plasmon polaritons (SPPs) are generated by the ring grating and couple with localized surface plasmons (LSPs) at the nanoantennas exciting emitters placed in the gap. We provide a thorough characterization of the optical properties of the simple ring grating structure, the double bowtie nanoantenna, and the integrated ring grating/nanoantenna structure, and study the coupling with an ensemble of molecules as well as single SiV centers in diamond. The combination of the sub-wavelength confinement of LSPs and the high energy of SPPs in our structure leads to precise nanofocusing at the nanoscale, which can be implemented to study plasmon-emitter coupling in the weak and strong coupling regimes.

Résumé: In the seminar I will discuss some of the elctromagnetic properties of viscous charged fluids. In particular I will show how the viscosity and the charge density of such media conspire to generate a negative refractive index for frequency below a certain characteristic frequency, for all hydrodynamic charged systems. I will then discuss possible application of this result for actual experimental setups, focusing on the case of electrons in in metals. I will provide for them a phenomenological model based on the idea of viscous, chardged dissipative fluid. The analysis of this model will confirm that finite viscosity leads to multiple modes of evanescent electromagnetic waves at a given frequency, one of which is characterized by a negative index of refraction. Bulding on this result I will discuss how optical spectroscopy can be used to probe the viscosity of electrons in metals, a concept very rarely discussed in theory and not measured yet in experiments. I will then show that finite viscosity lead to decrease the reflectivity of a metallic surface, and it provides charactristic sharp signatures in the reflection, refraction and transmission of electromagnetics waves through metallic samples. I will conclude commenting on the experiments that are presetly trying to probe negative refraction and viscosity in metallic samples. I will moreover provides some comments on possible underlying reasons why such general group of materials manifest negative refraction, and argue about the importance of spatial non-locality for this to happen.

Résumé: We appear opaque because our tissues scatter light very strongly. Traditionally, focusing of light in biological tissues is confounded by the extreme scattering nature of tissues. Interestingly, optical scattering is time-symmetric and we can exploit optical phase conjugation methods to null out scattering effects. I will discuss our recent results in using different types of guidestar methods in combination with digital optical phase conjugation to tightly focus light deep within biological tissues. These technologies can potentially enable incisionless laser surgery, targeted optogenetic activation, high-resolution biochemical tissue imaging and more.
Fourier Ptychography - Microscopes are complex and fussy creatures that are capable of delivering limited image information. This is because physical optical lenses are intrinsically imperfect. The perfect lenses we draw in high school ray diagrams simply do not exist. I will discuss our recent work on Fourier Ptychographic Microscopy - a computational microscopy method that enables a standard microscope to push past its physical optical limitations to provide gigapixel imaging ability.

Résumé: The tremendous progress in designing and tailoring the electric field in nano-resonators requires an investigation tool that is able to access the detailed features of the optical localized resonant modes with deep-subwavelength spatial resolution. This scenario has motivated the development of different nanoscale imaging techniques. Scanning near-field optical microscopy (SNOM) photoluminescence is proven to be a powerful technique. However it cannot be used for phase mapping and it is hard to be extended to silicon or polymer photonics
Here, we show that a technique involving the combination of scanning near-field optical microscopy with resonant scattering spectroscopy enables imaging the electric field in nano-resonators with outstanding spatial resolution (λ/19) by means of a pure optical method based on light scattering. By exploiting the Fano line shape, we can locally measure the phase modulation of the resonant modes without the need of external heterodyne detection. Also vectorial mapping is demonstrated. Finally we apply the method to the study of disorderd photonic system where ligth localization is directly imaged at the nanoscale.

Résumé:
The efficient absorption of audible sound takes an important position since excessive noise exposure becomes a major
public health concern. Thus, thin and lightweight absorbers that are both easily installed and capable to absorb sound over a wide
frequency range are strongly desired. In the first part of this seminar, I will present some of our recent activities in this field focusing
on the perfect acoustic absorption through the interplay of the inherent losses and transparent modes with high Q factor.
These modes are generated in a two-port, one-dimensional waveguide, which is side-loaded by isolated resonators of moderate Q factor.
In asymmetric structures, near perfect one-sided absorption has been observed (96%) with deep sub-wavelength samples.
Granular media are the second most manipulated material by man. However, the propagation of sound in granular solids remains a challenging scientific task. Important features of these media are: (i) the disorder in their packing and (ii) their nonlinear response.
In the second part of this seminar, I will present our investigations about energy transport in polydisperse granular chains.
After establishing the regime of sufficiently strong disorder, we focus our studies on the role of nonlinearity as this is desribed by Hertzian
contact mechanics. By increasing the initial excitation amplitudes we are able to identify three distinct dynamical regimes with different energy transport properties: a near linear, a weakly nonlinear and a highly nonlinear regime. We demonstrate that in the highly nonlinear regime, the
the energy is almost ballistically transported through shock-like excitations.

Résumé: Wave mixing is one of the most basic nonlinear optical processes. In the vast majority of cases, it is studied for one or more interacting quasi-monochromatic waves, where all temporal and spatial scales are large and similar. In contrast, there are very few studies of the mixing of wave packets having different wavelengths, spectra, and temporal and spatial profiles. Indeed, these configurations involve rather a complicated and non-intuitive wave mixing process such that it is difficult to assess, a priory, what would be the final spatial length, temporal duration and spectral width of the pulses generated by the interaction.
In this talk, I will describe one of the first thorough studies of such complex configurations, and present a systematic way to predict and interpret the wave mixing process in terms of exchange of spectral and spatial Fourier components between the interacting pulses. I will focus on a simple example where an intense short pump pulse is periodically-patterned such that it induces a transient Bragg grating (TBG) whose stop gap matches the frequency of an incoming long signal pulse.
I will demonstrate the validity of our interpretation using exact numerical simulations as well as a novel derivation of coupled mode theory for pulses propagating in time-varying media. Unlike previous attempts to derive such a model, our approach involves no approximation, and does not impose any restriction on the spatio-temporal profile. Moreover, the effect of modal dispersion on mode evolution and on the coupling to other modes is fully taken into account. It also avoids various artifacts of previous derivations by introducing the correct form of the solution, and can be applied to any other wave system.
I will then discuss the advantages and limitations of the proposed approach and demonstrate it for generating, switching and reversing ultrashort pulses propagating in several material platforms, such as silica fibers, semiconductor waveguides and plasmonic waveguides.

Résumé: Functional ultrasound imaging (US) has the potential to provide a high resolution image of the brain. However, finding functionally relevant structures can draw limitations to this resolution if we rely on matched anatomical atlases. Those limitations can arise from the amount of regional variations across individuals (this is large in human brain and not so large in rodent brain). They can also come from the way those atlases were produced, ie. based on anatomical markers or functional observations that are not relevant to the functions we investigate. Thus, to fully benefit from our resolution and achieve a relevant functional mapping, we need to apply data driven analysis and unsupervised learning methods such as Independent Component Analysis (ICA) and Deep Neural Networks (DNN). One way to go is to identify independent signal sources in each experiment and the corresponding activations. ICA is commonly used in fMRI and here I'll show that it's equally beneficial in fUS. Resting state activity was recorded in rodents during anaesthesia. Cortical and subcortical areas were reproducibly delineated at a fine scale at group level and back-projected to individual level. Correlation maps across areas delineated with ICASSO method reveal a strong correlation in the midline as well as local and contralateral connections. Using sparse autoencoders and DNNs with stacked layer configurations resulted in improved quality maps. The latter method was able to distinguish individual barrel activations in a whisker stimulation paradigm. The technique provides access to analysis at higher resolution and in paradigms when one cannot estimate the timing of the studied activations.

Résumé: At low temperature, liquid He transitions into a superfluid state as a result of macroscopic quantum coherence. By confining liquid 4He in well-defined structures of size comparable with the coherence length, nonbulk phenomena can be revealed. For instance, nanofluidic confinement has allowed the study of finite-size effects near the superfluid transition, realizing the most precise test of scaling laws to date. In order to study the thermodynamic properties of confined liquid 4He, we have developed a new set of measurements based upon superfluid nanomechanical resonators. We made an ultrasonic analog of the Fabry-Perot resonator to measure the sound velocity of liquid 4He in a few micron gap, and a superfluid Helmholtz resonator to measure the superfluid fraction and the onset of quantum turbulence in nanofluidic channels. In addition, at very low temperature (T ∼ 10 mK) where the normal component is negligible, the mechanical quality factor of superfluid resonators can be extremely large (Q > 10 billions), which makes nanomechanical superfluid resonators very promising systems for the field of optomechanics and quantum information.

Résumé: Acoustic imaging methods are useful in order to localize sound sources, examine how these radiate sound, characterize the acoustic properties of materials, and analyze complex sound fields. These methods typically rely on measurements with an array of microphones in order to characterize thespatio-temporal properties of the sound field under study. This talk has an emphasis on spherical array processing and sparsity promoting methods based on Compressive Sensing (CS). CS constitutes an interesting alternative to classical least-squares approaches. We will discuss applications where these techniques can be useful, such as sound source localization, sound absorption estimation, and sound field visualization in enclosures.

Résumé: The geometric phase is a general concept appearing in several domains in Physics. In wave physics, one can observe their existence in polarization, but they exist also for oscillatory mechanical systems or in quantum mechanics. During this seminar, we will present some geometric phase examples and discuss about the general conditions for their existence. We will then address the geometric phase of polarized waves in scattering media and discuss their relation with anisotropies.

Résumé: The ear works as a remarkable sound detector. Hearing can indeed operate over six orders of magnitudes of sound-pressure levels, with exquisite sensitivity and sharp frequency selectivity to weak sound stimuli. Curiously, the ear does not work as a high-fidelity sound receiver, introducing in the auditory percept “phantom” tones that are not present in the sound input. In this talk, I will present micromechanical experiments at the level of the cellular microphone of the inner ear – the hair cell – whose function is to transduce sound-evoked vibrations into electrical nervous signals. I will discuss the origin of stiffness and drag of the mechanoreceptive hair bundle, a tuft of cylindrical protrusions that protrudes from the apical surface of each cell, and show that hair cells can power spontaneous oscillations of their hair bundles. Oscillations are thought to result from a dynamical interplay between mechanosensitive ion channels, molecular motors, and calcium feedback. We find that oscillations of the hair bundle allow the hair cell to actively resonate with its mechanical input at the expense of distortions with properties that are characteristic of hearing. Our results promote a general principle of sound detection that is based on nonlinear amplification by self-sustained “critical” oscillators in the inner ear, i.e. active dynamical systems that operate on the brink of an oscillatory instability called a Hopf bifurcation.

Résumé: Atherosclerosis is one of the most deadly diseases in the world, killing about 17 million people each year by its sequelae, myocardial infarction
and stroke. Atherosclerotic plaques are complex, heterogeneous structures and it is difficult to predict which lesion will precipitate a clinical event.
Patient and lesion-specific assessment of the disease could provide better
guidance to therapeutic interventions, mechanical or pharmaceutical. In this talk, I will present some of our recent work on the development of new
technologies for imaging coronary and carotid atherosclerosis, based on
catheter-based optical coherence tomography at 5600 frames per second,
intravascular and non-invasive photoacoustic imaging, and imaging
submicron displacements with ultrasound.

Résumé: The recent realization of topological phases in insulators and superconductors has given incentive to explore analogous realizations. Propagation of microwaves in an array of dielectric ceramic cylinders constitutes a flexible experimental platform to investigate properties of topological states. I will show how a defect state in a dimerized chain of resonators can be topologically protected against structural disorder and how this state can be selectively enhanced by combining topological protection with non-hermitian symmetries. Our microwaves experiment contributed to create the active field of `artificial graphene'. I will present results related to the topological phase transition experiences by a honeycomb lattice under uniaxial strain: The gap opening and, depending of the boundary shape, the emergence of specific edges states.

Résumé: Transverse Anderson localization is the trapping of waves due a disordered potential which is invariant along propagation direction, canceling the effects of diffraction. It has recently been demonstrated that such form of wave trapping may be exploited to fabricate a novel generation of optical fibers which may either be obtained in plastic or in glass.
Here we report the effects of nonlinearity in disordered optical fibers supporting transverse Anderson localization. The presence of localization alters the response of a disordered plastic material turning a self-defocusing behavior into a self focusing one. At high optical power, modes are found reduced in extension and multiple optical beams inside this nonlocal disordered medium are subject to a form of long range interaction.
Moreover, we performed wavefront shaping experiments in disordered optical fibers in the transverse Anderson localization regime. By wavefront shaping and optimization, we observed the generation of a propagation-invariant beam, where light is trapped transversally by disorder. These localized states can be excited by extended speckled beams, and activated at a user defined target position, with a higher efficiency with respect to homogeneous (non disordered) systems.

Résumé: In complex media such as white paint or biological tissue, light encounters nanoscale refractive-index inhomogeneities that cause multiple scattering. It has been considered for a long time as a serious hitch to perform microscopy at depth, as traditional techniques rely on ballistic photons (which intensity exponentially decreases with depth).
In the last decade though, wavefront shaping emerged as a powerful tool to overcome this limitation. By controlling the phase pattern of the incident beam, one has been able to focus light through such an opaque highly scattering sample. Applications in imaging as well as in multimode fiber transmission control thus arose in the past few years.
After presenting the basics of light scattering and wavefront shaping, we will focus on two (very) different applications. We will first present how the propagation of non classical photon light can be manipulated in such scattering media. Then we will present how to improve imaging depth in biological samples.

Integrated in silico and in vivo approaches in the development of neuromodulation protocols in epilepsyJulien Modolo (Laboratoire de Traitement du Signal et de l'Image (LTSI), Université de Rennes 1)mardi 22 septembre 2015, 11:00, Amphi IPGP

Résumé: Neuromodulation, or the alteration of neuronal activity using electric, magnetic or chemical means, is becoming a therapy of growing importance in the treatment of neurological disorders. Parkinson’s disease is the best example of success of neuromodulation therapy, with over 100,000 patients implanted worldwide with a deep brain stimulation (DBS) device. In the field of epilepsy, neuromodulation therapy holds great promise for the symptomatic treatment of epilepsy, with encouraging results from studies conducted in epileptic patients using a chronically implanted device (RNS, Neuropace, USA) resulting in a mean decrease of 50% of seizures in 50% of patients. However, the mechanisms of action are still elusive, and there is still room for optimization of neuromodulation therapy in epilepsy. In order to address these issues, we are presenting an integrated approach combining electrophysiological and metabolic recordings in a mice model of epilepsy (kainate model) on the one hand, and in silico biophysical model of interictal and ictal activity on the other hand. We present experimental results on local electrical stimulation of the brain in epileptic mice, which suggest a possible decrease of pathological hyperexcitability of brain tissue. We then explore possible biological mechanisms using an in silico model of neuronal activity, which provides further insights in our experimental results. The perspectives of this work in terms of therapeutic potential and disease prevention will be presented.